1. Field of the Invention
The present invention relates to a method for improving the quality of bottom and water in closed water areas such as river mouths, lakes, ponds, bogs, etc., where water does not flow or flows very gently, and also relates to a set of agents used in the method.
2. Description of the Prior Art
As is generally known, sewage, and dirty water used in agricultural and industrial processes flow into closed water areas such as river mouths, lakes, ponds, bogs, etc., where water flows very gently. And in such closed water areas, because of mud deposits on the bottom, the water quality deteriorates, thereby presenting the problem of adversely affecting the ecosystem. In order to counteract the above problem, the sludge is either dredged or covered by sand. Alternatively, a sludge precipitation agents composed of zeolite processed in a special manner, or lime (quick lime or granular slaked lime) is spread.
However, when adopting the dredging method, the disposal of the sludge after being dredged out and a huge construction expense are the problems. Moreover, when dredging out, since the sludge is stirred at the bottom, the water quality there becomes even worse. On the other hand, when adopting the method of covering the sludge by sand, since the bottom surface becomes higher, shipping lanes may be disturbed, and again the huge construction expense is required. Moreover, sand or mud deposits may be spread out.
In the last method mentioned, the sludge precipitation agent composed of zeolite processed in a special manner is too expensive to be practically used.
A method of spreading granular slaked lime was presented by inventors of the present invention ("Environmental Conservation Engineering", Search Institute of Environmental Technology, Vol. 17, No. 12, Dec. 30, 1988, Vol. 18, No. 6, Jun. 30, 1989, Vol. 19, No. 5, May 30, 1990, and Vol. 19, No. 8, Aug. 30, 1990). As presented, when granular slaked lime is spread (200 g/m.sup.2), lime reacts with hydrogen sulfide in water and sludge, and hydrogen sulfide is destroyed. A calcium sulphate film was formed on the mud deposits, and this film serves as a barrier between mud deposits and water. This method enables various materials to be prevented from eluting into water, which may deteriorate water quality (hereinafter referred to as lime spreading method).
In the lime spreading method, only quick lime or granular slaked lime is spread. Therefore, this method has an advantage of low cost. This method is also beneficial for raising fish and shellfish by releasing Ca ions. Because of the above two advantages, the lime spreading method has been viewed with interest. However, this method also presents the following problems: water remains cloudy for several days after spreading lime (will be clear later); a long time is required for improving water (for example, a long time is required for reducing a BOD value); a long time is required for precipitating suspended matters in water; and decomposition of the sludge isolated from the water by the calcium sulphate film is left to the natural phenomenon.